1. Field of the Invention
The present invention relates to a control method for a coordinated regenerative brake-system in an electric vehicle.
2. Description of Related Art
Electric vehicles having an electric motor or other electrical drive source as the drive source creating drive power for the vehicle typically have a brake system that uses both frictional brake torque produced by a hydraulic mechanism of some kind, and regenerative brake torque produced by converting some of the vehicle""s kinetic energy to electrical energy that is stored in the vehicle""s battery. It is to be noted that xe2x80x9celectric vehiclexe2x80x9d as used in this specification includes so-called hybrid vehicles that are capable of using drive power from an internal combustion engine as well as electrical drive power.
When brake torque is applied to the wheels using both friction and regenerative brake torque, coordinated regenerative braking is used to coordinate the two braking mechanisms so that the timing and balance of torque from the two systems is good and the total brake torque developed is in accordance with the brake torque intended by the driver. In other words, coordinated regenerative braking blends the brake force developed by the two systems so that operation is transparent to the user and corresponds to the driver""s operation state of the brakes (such as how far the brake pedal is depressed). When the total required brake torque is less than the maximum achievable regenerative brake torque, which depends on vehicle speed and battery capacity, coordinated regenerative braking supplies the required brake torque using only the regenerative brake system, but when the required brake torque exceeds this limit, that is, exceeds the maximum achievable regenerative brake torque, the deficient part of the required brake torque is produced using the friction brake system.
Various measures have been taken in electric vehicles equipped with a coordinated regenerative brake system to eliminate differences in the feel between the electric vehicle and a conventional engine-powered vehicle so as to eliminate any driver discomfiture and resistance to driving an electric vehicle resulting from an unaccustomed feel in brake behavior. These measures included developing a method for controlling the coordinated regenerative braking system during vehicle braking so that braking an electric vehicle feels as much as possible identical to braking a conventional engine-powered vehicle.
For example, with a conventional vehicle it is possible to apply brake the vehicle by operating the brake pedal, or by applying a so-called engine brake by letting up on or completely releasing the accelerator pedal under specific conditions so that it is not depressed at all. An engine brake is commonly used when, for example, travelling down a relatively steep, long slope or applying the brakes continuously for a relatively long period of time.
To make braking feel qualitatively the same in conventional and electric vehicles, methods for controlling the coordinated regenerative braking system so as to produce brake torque equivalent to the engine brake produced during normal vehicle deceleration have been proposed. See, for example, Japanese Patent Laid-Open Publication Hei 6-153315. In this prior art, brake torque equivalent to the engine brake is given by regenerative brake torque as the brake torque in accordance with the operational state of accelerator pedal.
Problem to be Solved
It is commonly known that brake torque comparable to engine brake torque in a conventional vehicle decreases as vehicle speed and engine speed decrease. When using regenerative brake torque, however, it is possible to maintain a constant brake torque comparable to engine brake torque to a very low speed when compared with a conventional engine-powered vehicle.
If, however, braking with this constant torque continues to the extremely low speed at which regeneration is possible, this constant brake torque will be applied until immediately before the vehicle stops. This makes it very difficult to come to a smooth, natural stop at a particular position in the same way that a conventional vehicle can. Furthermore, regenerative brake torque goes substantially to zero at the regeneration limit speed immediately before the vehicle stops, resulting in a sudden discomfiting loss of deceleration that produces driver discomfiture and resistance to braking, also makes working the brakes to come to a stop at a specific point difficult.
To resolve the above noted problems, the invention provides a method for controlling the coordinated regenerative braking system of an electric vehicle so that total brake torque is applied to the vehicle in accordance with brake pedal operational state, thereby assuring that vehicle deceleration is commensurate with the braking intention of the driver when the accelerator pedal is not depressed during vehicle deceleration.
As illustrated in FIG. 7, the invention is based on setting brake torque Ta, which is equivalent to the engine brake torque to be applied to the vehicle when accelerator pedal operation amount Sa (accelerator pedal depression) is zero, so that it decreases gradually according to vehicle speed V in a vehicle speed range below a specific speed V1, which is set to a relatively low speed such as approximately 20 km/h.
However, when brake torque Ta is set as described above, and the driver works the brake pedal to achieve a desired vehicle deceleration after letting up on or completely releasing the accelerator pedal so that accelerator pedal depression Sa is actually zero (i.e., is not depressed at all), engine brake equivalent torque Ta decreases in accordance with vehicle speed V regardless of driver intentions in the vehicle speed range below or equal to specific speed V1. It is therefore not possible to achieve vehicle braking as desired by the driver.
In the example shown in FIG. 7 the sum of brake torque Tb, which corresponds to brake pedal depression Sb, and brake torque Ta (that is, torque equivalent to the engine brake torque), which corresponds to accelerator pedal depression Sa, is normally the desired total brake torque Tq to be developed and applied to the vehicle. However, because engine brake equivalent torque Ta decreases gradually according to vehicle speed V in the vehicle speed range below specific speed V1 even if the driver keeps brake pedal depression Sb constant in order to achieve constant vehicle deceleration, the total brake torque Tq acting on the vehicle decreases in conjunction with the drop in torque Ta, and vehicle. deceleration decreases contrary to the driver""s intention. This means that it is necessary to further depress the brake pedal in order to maintain the constant vehicle deceleration desired by the driver. This can be discomfiting and unnatural to a driver accustomed to driving a conventional engine-powered vehicle, and makes vehicle braking and deceleration control difficult.
To resolve this problem, a first aspect of the present invention provides a control method for a coordinated regenerative brake system in which the total brake torque to be applied to a vehicle is produced by coordinating friction brake torque and regenerative brake torque, and specifically controls [a] in a vehicle speed range less than or equal to this specific value (V1), when accelerator operator operation amount (Sa) is effectively zero, a brake torque (Ta) equivalent to engine brake torque working on the vehicle is set to be controlled to approach gradually zero in accordance with vehicle speed (V). Furthermore, when [b] brake operator operation starts in a vehicle speed range exceeding the specific value (V1), brake torque is controlled [b1] using as a desired total brake torque (Tq) for the vehicle in the vehicle speed range exceeding the specific value (V1), a sum of brake torque (Tb) corresponding to operational state of a brake operator, and brake torque (Ta) corresponding to operational state of accelerator operator. In [b2] the vehicle speed range less than or equal to the specific value (V1), [b3] using as the desired total brake torque (Tq) the sum of brake torque (Tb) corresponding to operational state of a brake operator, and first engine brake equivalent torque (Ta1), which is comparable to engine brake torque when vehicle speed (V) reaches the specific value (V1), when brake operator operation amount (Sb) is equal to or greater than a first brake operator operation amount (Sb1), which is brake operator operation when the vehicle speed (V) reaches the specific value (V1); and [b4] using as the desired total brake torque (Tq), when brake operator operation amount (Sb) is less than the first brake operator operation amount (Sb1), the sum of brake torque (Tb) corresponding to operational state of a brake operator, and torque (Taxe2x80x2) calculated to approach brake torque (Ta), which corresponds to operational state of an accelerator operator, from first engine brake equivalent torque (Ta1) in accordance with the reduction ratio of brake operator operation amount (Sb).
According to the first aspect of the invention, by controlling brake torque Ta, which is equivalent to the engine brake torque in a conventional vehicle, so that it gradually goes to zero in accordance with the drop in vehicle speed V in a vehicle speed range equal to or below a specific speed (V1), which is set to a relatively low speed range, it is possible to improve problems presented by the related art, and achieve relatively natural deceleration and stopping comparable to a conventional engine-powered vehicle. As noted above, these problems include difficulty operating the brakes to stop at a particular point when a constant brake torque equivalent to engine brake torque continues working until immediately before the vehicle stops, and the sudden loss of deceleration that occurs when regenerative brake torque (equivalent to engine brake torque) goes suddenly to zero at the regeneration limit speed immediately before the vehicle stops.
Furthermore, with keeping such a effect, brake torque is controlled using as the desired total brake torque (Tq) of the vehicle the sum of brake torque (Tb) corresponding to operational state of a brake operator, and brake torque (Ta) corresponding to operational state of an accelerator operator, when operation of the brake operator begins in the vehicle speed range above the specific speed V1. It is therefore additionally possible to apply total brake torque to the vehicle in accordance with operational state of the brake operator (that is, in accordance with the braking intended by the driver).
Yet further, in the vehicle speed range equal to or below the specific speed V1, brake torque is controlled using as the desired total brake torque (Tq) of the vehicle the sum of brake torque (Tb) corresponding to operational state of a brake operator, and first engine brake equivalent torque (Ta1), which is comparable to engine brake torque at the specific speed (V1), if brake operator operation amount (Sb) is equal to or greater than a first brake operator operation amount (Sb1) at the specific speed (V1). As a result, a drop in vehicle deceleration not in accordance with the driver""s intention can be prevented even in the vehicle speed range below specific speed V1, and total brake torque can be applied to the vehicle as desired by the driver.
Yet further, when brake operator operation amount (Sb) is less than a first brake operator operation amount (Sb1), brake torque is controlled using as the desired total brake torque (Tq) of the vehicle the sum of brake torque (Tb) corresponding to operational state of a brake operator, and torque (Taxe2x80x2) calculated to approach brake torque (Ta) corresponding to operation of an accelerator operator from first engine brake equivalent torque (Ta1) in accordance with the reduction ratio in brake operator operation amount (Sb) It is therefore possible in this case, too, to faithfully brake as intended by the driver, and achieve relatively natural vehicle deceleration and stopping.
Further preferably, in a second aspect of the present invention, when brake operator operation starts in a vehicle speed range less than or equal to the specific value (V1), brake torque is controlled [c] using as the desired total brake torque (Tq), while brake operator operation amount (Sb) is increasing, a sum of brake torque (Tb) corresponding to operational state of a brake operator, and second engine brake equivalent torque (Ta2), which is equivalent to engine brake torque at the start of brake operator operation; and [d] using as the desired total brake torque (Tq), when brake operator operation amount (Sb) decreases, a sum of brake torque (Tb) corresponding to operational state of a brake operator, and torque (Taxe2x80x3), which is calculated to approach brake torque (Ta) corresponding to operational state of an accelerator operator from second engine brake equivalent torque (Ta2) in accordance with the reduction ratio of brake operator operation amount (Sb) from a relative maximum operation.
According to the second aspect of the invention, in addition to achieving the benefits as described above of a control method according to the first aspect of the invention, also controls brake torque using as desired total brake torque (Tq) of the vehicle the sum of brake torque (Tb) corresponding to operational state of a brake operator, and second engine brake equivalent torque (Ta2), which is equivalent to the engine brake torque at the start of brake operator operation, when brake operator operation starts in the vehicle speed range below the specific speed V1 and brake operator operation amount (Sb) is increasing. It is therefore possible even in the vehicle speed range below the specific speed V1 to prevent a drop in vehicle deceleration not in accordance with the driver""s intention, and apply total brake torque to the vehicle corresponding to the braking as intended by the driver.
Furthermore, when brake operator operation amount (Sb) decreases, brake torque is controlled using as the desired total brake torque (Tq) the sum of brake torque (Tb) corresponding to operational state of a brake operator, and torque (Taxe2x80x3), which is calculated to approach brake torque (Ta) corresponding to operational state of an accelerator operator from second engine brake equivalent torque (Ta2) in accordance with the reduction ratio in brake operator operation amount (Sb) from a maximum operation limit. It is therefore possible in this case, too, to faithfully brake as intended by the driver, and achieve relatively natural vehicle deceleration and stopping.
Other objects and attainments together with a fuller understanding of the invention will become apparent and appreciated by referring to the following description and claims taken in conjunction with the accompanying drawings.